This invention relates to electric motors and more particularly to an improved switched reluctance motor construction.
Switched reluctance motors have attracted considerable attention over the past ten to fifteen years, primarily due to the simplicity of their construction and high power densities (ratio between output power and weight). These motors are doubly salient motors, having teeth on both the stator and the rotor, with phase windings only on the stator poles.
Except for very small (below 50 watt) motors, most switched reluctance motors are designed to operate below 2000 RPM because the core losses in switched reluctance motors are several times larger than in conventional machines of the same size. For that reason, conventional wisdom is that switched reluctance motors are best suited for low speed applications. Of course certain applications such as air cooling compressor motors are required to operate at much higher speeds, such as 6000 RPM.
Early switched reluctance motors for larger power levels had six poles on the stator and four on the rotor. To reduce the number of power switches required to supply power to these motors, later designs adopted an 8/6 construction (eight stator poles and six rotor poles), which required fewer power switches and had improved starting torque.
In order to limit the core losses in prior machines, the direction of the stator magneto-motive force (mmf) was selected in such a way that the mmf direction in the rotor changed only once per full rotor revolution. In this way, the high frequency flux changes, which are proportional to the number of rotor poles, appeared only in the stator poles and the edges of the rotor poles. As a result, the core losses in the stator yoke and rotor core were reduced at the expense of some torque imbalance. Of greater concern are the losses in the stator poles due to flux bypass with such a construction. This bypass flux produces a torque in the opposite direction, and the bypass mmf has the opposite direction from the main mmf to be established when the next phase is energized. This change in the mmf direction in the stator poles of prior motors increased the range of flux variation and lead to increased core losses in stator poles.
In addition to the normal losses due to eddy currents and hysteresis, core losses are also affected by the method conventionally used in stamping the rotor and stator laminations. Stamping the laminations for conventional machines is done as follows: First the stator and rotor slots are stamped out and then the rotor lamination is stamped from the stator lamination. As a result, both stator and rotor teeth are sharply rectangular.
Apparently the laminations for switched reluctance motors up to now have been made in the same way. As a result, the stator and rotor poles or teeth have sharp, rectangular corners. Since switched reluctance motors have only one tooth per pole and since they operate on the attraction between teeth/poles, there is a very strong flux concentration at the corners of each pole, prior to and after the alignment of the stator and rotor poles, resulting in increased losses. Rounding of these corners would, on the other hand, appear to require a new method of lamination stamping.
Furthermore, the stamping method presently used on conventional machines requires final machining of the rotor surface, to obtain the exact rotor diameter. This machining normally results in short circuiting of some of the rotor laminations, as does the welding used to hold the laminations together. Although the frequency of the rotor flux in induction machines is low, this manufacturing method and the resulting short-circuiting of laminations causes additional losses which may amount to several percent of the total losses. In switched reluctance motors the machining necessary to obtain the exact rotor diameter would considerably increase the total losses given the frequency change of the rotor flux variations.
Finally, large leakage flux when the rotor is in the position of maximum magnetic reluctance results in flux lines perpendicular to the rotor and stator surfaces. Rectangular poles make the flux lines longer, further increasing the losses.
For the above reasons, most of the switched reluctance motors used up to now have high quality laminations, with a thickness of 0.014" which is smaller than in conventional machines. Furthermore, in order to reduce vibrations caused by the strong changes in mmf, present switched reluctance motors have rotor lamination assemblies which are bonded together by adhesives.
Reducing the number of rotor poles is advantageous in designing switched reluctance motors for operation at higher speeds. However with conventional rotor construction, it is not feasible to reduce the number of rotor poles below four. Even with four poles, the flux density (and thus the losses) is high due to the opening in the lamination stack for the motor shaft, which decreases the effective cross-section of the rotor core.
For example, a three-pole rotor in a switched reluctance motor has no room for a conventional shaft if the flux density in the rotor core is to be held at an acceptable level. At the same time, due to the odd number of rotor poles, the radial forces with a three-pole rotor are very large and unbalanced, requiring an even larger shaft than normal. These very strong one-directional radial forces require an exceptionally stiff rotor construction.